Various techniques exist for synchronous, bit-serial digital communications. Because both clock and data are sent over the same communications channel in such systems, particular attention must be (and is) given to protecting the integrity of the signal thus conveyed.
One such technique is known in the art as Manchester encoding. To produce Manchester encoded data, representations of digital data bits and a clocking signal are combined together for transmission as but one common information signal which may be sent over a single serial data channel. Frequently, a coaxial cable constitutes the transmission medium of the information channel. A decoder, coupled to the information channel at the receiver, separates the data bits and clocking signals from the composite signal, whereby the data bits, under control of the clocking signals extracted thereby, are transferred to the device which is coupled to the decoder. With most coding schemes, more difficulties lie with decoding operations than with encoding operations, and Manchester-type encoding follows this general rule. See, for example, U.S. Pat. Nos. 4,167,760 and 4,317,211. This invention, too, is directed to the provision of an efficient Manchester-type decoder.
However, this invention is directed in particular to the problems encountered when Manchester or other self-clocking encoding is used for serial transmission over a carrier-sense multiple access (CSMA) channel. Since, in a communications system employing Manchester-type codes, the timing signals needed for controlling the transfer of data bits at the receiving end are derived both from clocking signals transmitted by a transmitting device (i.e., they must be extracted from the data channel) and from an internal clock source in the receiving device, some means must be provided to synchronize the two clocks. (It is presumed the transmitting device operates asynchronously and independently from the receiving device.) Furthermore, the external clocking signals transmitted by the transmitting device may become corrupted by, for example, noise or a collision of signals on the channel due to two or more transmitting devices attempting simultaneously to transmit information. It is desirable, therefore, to prevent collisions or other corruptive influences from interfering with internal timing operations.
Further, as with any digital data communication system, it is desired to attain as high a rate as possible of data transfers, keeping within the cost and timing contraints imposed by available circuit components. It is an object of this invention to provide for very high speed serial transfers without incurring exhorbitant circuitry costs. Prior art systems which might use, for example, a phase-locking loop circuit for controlling the Manchester decoding operations are both costly and relatively slow in locking onto information signals transmitted at rates of, say, 70 megabits per second or more.
A further objective of this invention is to provide an interface for coupling a computer with a data communications link that permits asynchronous bit-serial communications among devices transmitting and receiving Manchester-type codes over a serial information channel.
Another objective of this invention is to provide an economical and efficient Manchester decoder useful for decoding high-speed bit-serial information transmitted over a serial communications link that is shared by several devices connected thereto.
Another objective of this invention is to provide means in the interface for detecting the presence of a carrier signal on the serial data channel.
Yet another objective of the present invention is to provide means for synchronizing an internal clock source with an external clock source derived from a Manchester encoded signal transmitted by another device, with means for preventing collisions from interfering therewith.